This invention relates to a stepping motor control apparatus for controlling the stepping motor, and especially relates to a stepping motor control apparatus for enabling the stepping motor to be intermittently driven during a micro transfer period obtained by dividing regular step transfer period.
In the stepping motor, it is comparatively easy to position a rotor in good accuracy and is easy to execute digital control thereof so that the stepping motor is utilized in various application fields and the control thereof is carried out as follows.
This stepping motor is driven by switching on and/or off exciting current of motor-coils. Namely, if two-phase stepping motor is driven via bipolar stator, the exciting currents of coils A and B are respectively controlled by switching on and/or off the exciting currents in accordance with the current patterns 0, 1, 2 and 3 of the Table 1.
TABLE 1 ______________________________________ CURRENT PATTERN 0 1 2 3 ______________________________________ COIL A + 0 - 0 B 0 + 0 - ______________________________________
FIG. 2 illustrates a static torque characteristic Tr of this stepping motor and the torque Tr is a function Tr(.theta.) of angular position .theta. of the rotor. In FIG. 2, the maximum torque of the torque Tr is designated by the value Tr.sub.0 and the torque characteristic Tr shows generally sine wave characteristic having a variable of the rotor angular position .theta.. At angular positions .theta..sub.0, .theta.'.sub.0, the rotor of the stepping motor stably stops, and thus each position becomes each detent position.
Namely, when some external torque is occasionally applied to the rotor which is stopped in one of the detent positions, the reaction torque toward the reverse direction, that is, toward the detent position, is induced. For example, when some external torque is applied to the rotor at the position .theta..sub.0 in the direction of the position .theta..sub.1, positive reaction torque is induced and when some external torque is applied to the rotor at the position .theta..sub.0 in the direction of the position .theta..sub.2, the negative reaction torque is induced and thus, the rotor is stably converged toward the detent position.
As a current pattern corresponding to the detent position .theta..sub.0, the pattern 0 in the Table 1 is selected and the torque characteristic of this current pattern 0 is shown in FIG. 3 by Tr.sub.0 (.theta.).
Additionally, in FIG. 3, each torque characteristic of current patterns 1, 2 and 3 is shown by the curves Tr.sub.1 (.theta.), Tr.sub.2 (.theta.) and Tr.sub.3 (.theta.) respectively and it is understood from this figure that if the current pattern is successively changed to 0, 1 and 2 in order and/or changed to 2, 1 and 0 in order, the rotor is successively transferred to detent positions .theta..sub.00, .theta..sub.01 and .theta..sub.02 in order and/or transferred to .theta..sub.02, .theta..sub.01 and .theta..sub.00 in order.
Accordingly, by controlling the current pattern as set forth above, that is, by executing the on-off control of the exciting currents of the motor-coils A and B, the rotor is intermittently driven toward the forward direction or the reverse direction of the angular position .theta. in FIG. 3. In case such a regular stepwise driving, the rotor can be stably hold at each detent position and this manner is obvious from FIG. 4 (in this figure, the rotor is shown that it is driven from the detent position .theta..sub.00 to the detent position .theta..sub.01).
As mentioned above, by executing the on-off control of the exciting current of each motor-coil of the stepping motor, it becomes possible that the rotor is controlled so as to be successively transferred in a regular stepwise manner from the present detent position to the next detent position. In addition, the holding torque at the detent position is so large that it becomes possible to carry out high accurate positioning control of the rotor at high speed in spite of the open-loop control.
But, in a stepping motor, the step number per one rotation of the rotor (the detent position number) is in the range from tens to hundreds and is one thousand even in its maximum value such that it is difficult to execute finer positioning of the rotor.
Therefore, it is proposed to drive the stepping motor in a micro-stepwise or variable step manner and thereby, the rotor is intermittently driven during the micro-step transfer period through a variable angular transfer distance which is obtained from dividing a fixed angular interval between adjacent detent positions of a regular stepwise rotation such that the higher resolvable or finer positioning becomes possible.
In this micro-stepwise driving, the exciting current of the holding position defining a final rotor is derived from the predetermined wave form pattern (for example, a sine wave or a triangular wave) and each motor-coil is excited by these exciting currents. For example, in two phase driving, these exciting currents are not same each other and a desired holding position between adjacent regular detent positions is determined by these exciting currents.
For instance, if the fixed regular stepping transfer interval is divided by number n and an arbitrary holding position within the regular stepping interval is defined by m and a maximum exciting current is defined by I.sub.0, to the A coil of two-phase motor, an exciting current I.sub.0 cos (.pi.m/2n) is applied and to the B phase coil thereof, an exciting current I.sub.0 sin (.pi.m/2n) is applied. By the way, each of conditions m=1n, m=2n and m=3n corresponds respectively to each of the current patterns 1, 2 and 3 as shown in Table 1.
Furthermore, the micro- or variable step driving is described in conjunction with the characteristic diagram of FIG. 5. In this figure, when each exciting current is changed from the still pattern corresponding to m=m.sub.0 and .theta.=.theta..sub.0 to the pattern corresponding to m=m.sub.1 (.vertline.m.sub.1 -m.sub.0 .vertline..ltoreq.n) so as to allow to be led to the A phase and B phase, the torque characteristic thereof is changed from the characteristic Tr=Trm.sub.0 (.theta.) which is derived in case of m=m.sub.0, to the characteristic Tr=Trm.sub.1 (.theta.). Thus, in case of .theta.=.theta.m.sub.0, the rotor generates therein a torque Tr.sub.0 sin {.pi.(m.sub.1 -m.sub.0)/2n} so as to be transferred toward the holding position .theta.m.sub.1 in a micro-stepwise manner.
But, as the division number n becomes large and the magnitude m.sub.1 -m.sub.0 becomes small, the acceleration torque Tr.sub.0 sin {.pi.(m.sub.1 -m.sub.0)/2n} becomes smaller than the acceleration torque generated in case of regular stepping driving shown in FIG. 3 set forth above, and thereby the acceleration of the rotor in forward and reverse directions becomes sluggish. This drawback is illustrated by the characteristic curve 100 of FIG. 6 and the rotor is gradually transferred from present holding position .theta.m.sub.0 to the final holding position .theta.m.sub.1 and is stably converged to the final holding position .theta.m.sub.1 after the lapse of long time.
Accordingly, in a micro-stepwise or variable step driving of this kind, the positioning of the rotor becomes highly precise, but positioning control at high transfer speed becomes impossible and a control error will occur.